Due to their exposure to the open air because of the cable trays, the wires contained within need a very durable outer covering. The regulations dictate that the cables must either be Type TC (also known as Tray Rated) or must be metal-armored (Type MC). Cable tray types, fill rules for single-conductor and multiconductor cables, ampacity derating, separation requirements, and when to use tray vs conduit. Cable tray is the preferred wiring method for industrial facilities, data centers, and large commercial buildings where routing dozens or. Fill Limits: For power cables, the fill must not exceed 40% of the tray's cross-sectional area; for control cables, it's 50%. NEC section 300-8 does not permit. Cable trays are structural components of a facility's electrical system, and as such, are part of a planned cable management system. 305(a)(3) and within various provisions of the National Electric Code (NEC). The two most common methods to.